Serum Albumin
Serum albumin is the most abundant protein in mammalian sera (40 g/I; approximately 0.7 mM in humans), and one of its functions is to bind molecules such as lipids and bilirubin (Peters, Advances in Protein Chemistry 37:161, 1985). Serum albumin is devoid of any enzymatic or immunological function. Furthermore, human serum albumin (HSA) is a natural carrier involved in the endogenous transport and delivery of numerous natural as well as therapeutic molecules (Sellers and Koch-Weser, Albumin Structure, Function and Uses, eds Rosenoer et al, Pergamon, Oxford, p 159, 1977). The half life of serum albumin is directly proportional to the size of the animal, where for example human serum albumin has a half life of 19 days and rabbit serum albumin has a half life of about 5 days (McCurdy et al, J Lab Clin Med 143:115, 2004). HSA is widely distributed throughout the body, in particular in the interstitial and blood compartments, where it is mainly involved in the maintenance of osmolarity. Structurally, albumins are single-chain proteins comprising three homologous domains and in total 584 or 585 amino acids (Dugaiczyk et al, Proc Natl Acad Sci USA 79:71, 1982). Albumins contain 17 disulfide bridges and a single reactive thiol, cysteine in position 34, but lack N-linked and O-linked carbohydrate moieties (Peters, 1985, supra; Nicholson et al, Br J Anaesth 85:599, 2000).
Fusion or Association with HSA Results in Increased In Vivo Half Life of Proteins
Several strategies have been reported to either covalently couple proteins directly to serum albumins or to a peptide or protein that will allow in vivo association to serum albumins. Examples of the latter approach have been described e.g. in WO91/01743, in WO01/45746 and in Dennis et al (J Biol Chem 277:35035-43, 2002). The first document describes inter alia the use of albumin binding peptides or proteins derived from streptococcal protein G (SpG) for increasing the half life of other proteins. The idea is to fuse the bacterially derived, albumin binding peptide/protein to a therapeutically interesting peptide/protein, which has been shown to have a rapid elimination from blood. The thus generated fusion protein binds to serum albumin in vivo, and benefits from its longer half life, which increases the net half life of the fused therapeutically interesting peptide/protein. WO01/45746 and Dennis et al relate to the same concept, but here, the authors utilize relatively short peptides to bind serum albumin. The peptides were selected from a phage displayed peptide library. In Dennis et al, earlier work is mentioned in which the enhancement of an immunological response to a recombinant fusion of the albumin binding domain of streptococcal protein G to human complement receptor Type 1 was found. US patent application published as US2004/0001827 (Dennis) also discloses the use of constructs comprising peptide ligands, again identified by phage display technology, which bind to serum albumin and which are conjugated to bioactive compounds for tumor targeting.
Albumin Binding Domains of Bacterial Receptor Proteins
Streptococcal protein G (SpG) is a bi-functional receptor present on the surface of certain strains of streptococci and is capable of binding to both IgG and serum albumin (Björck et al, Mol Immunol 24:1113, 1987). The structure is highly repetitive with several structurally and functionally different domains (Guss et al, EMBO J 5:1567, 1986), more precisely three Ig-binding domains and three serum albumin binding domains (Olsson et al, Eur J Biochem 168:319, 1987). The structure of one of the three serum albumin binding domains in SpG has been determined, showing a three-helix bundle fold (Kraulis et al, FEBS Lett 378:190, 1996, Johansson et al, J. Biol. Chem. 277:8114-20, 2002). A 46 amino acid motif was defined as ABD (albumin binding domain) and has subsequently also been designated G148-GA3 (GA for protein G-related albumin binding). In for example WO09/016043, albumin binding variants of the 46 amino acid motif ABD are disclosed.
Other bacterial albumin binding domains than the ones in protein G have also been identified, some of which are structurally similar to the ones of protein G. Examples of proteins containing such albumin binding domains are the PAB, PPL, MAG and ZAG proteins (Rozak et al, Biochemistry 45:3263-3271, 2006). Studies of structure and function of such albumin binding domains have been carried out and reported e.g. by Johansson and co-workers (Johansson et al, J Mol Biol 266:859-865, 1997). Furthermore, Rozak et al have reported on the creation of artificial variants of G148-GA3, which were selected and studied with regard to different species specificity and stability (Rozak et al, Biochemistry 45:3263-3271, 2006), whereas Jonsson et al developed artificial variants of G148-GA3 having very much improved affinity for human serum albumin (Jonsson et al, Prot Eng Des Sel 21:515-27, 2008). For some of the variants a higher affinity was achieved at the cost of reduced thermal stability.
In addition to the three-helix containing proteins described above, there are also other unrelated bacterial proteins that bind albumin.
ABD and Immunization
Recently, a few T- and B-cell epitopes were experimentally identified within the albumin binding region of Streptococcal protein G strain 148 (G148) (Goetsch et al, Clin Diagn Lab Immunol 10:125-32, 2003). The authors behind the study were interested in utilizing the T-cell epitopes of G148 in vaccines, i.e. to utilize the inherent immune-stimulatory property of the albumin binding region. Goetsch et al additionally found a B-cell epitope, i.e. a region bound by antibodies after immunization, in the sequence of G148.
In pharmaceutical compositions for human administration no immune-response is desired. Therefore, the albumin binding domain G148 is as such unsuitable for use in such compositions due to its abovementioned immune-stimulatory properties.